Anti-spall lightweight armor

ABSTRACT

A shock absorbent layered combination of materials, comprising: an outer layer possessing a tensile strength of between 104 and 106 pounds per square inch and an elastic modulus of between 105 and 108 pounds per square inch; and a deformable inner layer rigidly adhered to said outer layer, wherein the material constituting said outer layer possesses both a higher tensile strength and a higher elastic modulus than the material constituting said inner layer.

United States Patent Gulbierz et a1.

1 1 Nov. 13, 1973 ANTLSPALL LIGHTWEIGHT ARMOR Inventors: Joseph E.Gulblerz; Charles F.

Bohan, both of Wharton, NJ.

[73] Assignee: The United States of America as represented by theSecretary of the Army, Washington, DC.

[22] Filed: Sept. 29, 1971 [21] Appl. No.: 184,845

3,135,645 6/1964 Burkley et a1. 4. 161/404 3,431,818 3/1969 King 89/36 A3,486,966 12/1969 Allen et a1. 161/404 2,758,952 8/1956 Toulmin t 89/36A 2,960,424 11/1960 Bjorholm.. 161/404 3,506,531 4/1970 Stander 161/93Primary ExaminerStephen C. Bentley Attorney-Harry M. Saragovitz et a1.

[57] ABSTRACT A shock absorbent layered combination of materials,comprising: an outer layer possessing a tensile strength of between 10and 10 pounds per square inch and an elastic modulus of between 10 and10'' pounds per square inch; and a deformable inner layer rigidlyadhered to said outer layer, wherein the material constituting saidouter layer possesses both a higher tensile strength and a higherelastic modulus than the material constituting said inner layer.

4 Claims, 4 Drawing Figures ANTI-SPALI. LIGHTWEIGHT ARMOR The inventiondescribed herein may be manufactured, used and licensed by or for theGovernment of the United States for governmental purposes without thepayment to us of any royalty thereon.

BACKGROUND OF THE INVENTION High hardness steel armor has been theconventional type of armor used on military vehicles for at least thirtyyears. Steel armor, while efficacious for its intended purpose, hasnontheless suffered from certain shortcomings. These include excessiveweight (technically known as areal density), difficulty of formationinto complex shapes, sensitivity to extremes of heat, and brittleness.

The problem of brittleness generates a further problem known asspalling. Spalling often occurs when tank armor is struck by small armsammunition of the type widely used by infantry personnel. Morespecifically, spalling is the high-velocity discharge of fragments froma tank's armor. This discharge occurs when the armor is struck atoblique angles and at ballistic speeds by ball-type ammunition. Thisclass of ammunition includes virtually every type of conventionalmunition except armor piercing artillery. Hence, a need has alwaysexisted for an armor that would reduce spall, both frontal and rear withits concommitant danger to nearby friendly troops and equipment, withoutsacrificing any of the protection which steel armor offers to the tankcrew. The present invention provides such an armor.

SUMMARY OF THE INVENTION An object of the present invention is toprovide a lightweight anti-spall armor for military vehicles.

Another object is to provide a high-impact absorbent layered combinationof materials whose shape can be readily conformed to that of a militaryvehicle such as a tank.

A further object is to provide a lightweight armor that is relativelyunaffected by the extremes of temperature encountered in a militaryenvironment.

A yet further object is to provide a multilayered armor wherein theinnermost layer can comprise a structural member of the protectedvehicle.

The present invention comprises: a shock absorbent layered combinationof materials, comprising: an outer layer possessing a tensile strengthof between l and I0 pounds per square inch and an elastic modulus ofbetween and I0 pounds per square inch; and a deformable inner layerrigidly adhered to said outer layer, wherein the material constitutingsaid outer layer possesses both a higher tensile strength and a higherelastic modulus than the material constituting said inner layer.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional side viewof the present anti-spall lightweight armor.

FIG. 2 is a microscopically enlarged cross-sectional view of the outerlayer of the armor.

FIG. 3 is a microscopically enlarged cross-sectional view of the innerlayer of the armor.

FIG. 4 is a representation, in cross-sectional view, of a train of shockwaves from a projectile passing into the present multi-layeredcombination.

DETAILED DESCRIPTION OF THE INVENTION Conventional steel armor of thetype popularly used on today's tanks possesses an areal density (weightper unit of surface area) of about 20 pounds per square foot. The needhas long been felt to not only effect a reduction of this densityfigure, but to also develop a suitable substitute armor that could bemore readily conformed to the outer structural members of a tank. Also,a less brittle armament material has long been sought.

The present invention utilizes a basically two layered configuration oflightweight material wherein each layer, taken alone is weaker, i.e.,lower tensile strength and elastic modulus, than hardened steel butwherein the two layers taken together perform the same function as steelbut at a markedly lower areal density, e.g., about 10 pounds per squarefoot.

FIG. 1 illustrates an outer layer I0. It is this outer layer 10 that ashock producing stimulus, such as small arm ammunition or a projectileI8, is incident upon. Also illustrated is a deformable inner layer 12which is adhered to an outer structural member 14 of a tank or militaryvehicle.

The outer and inner layers 10 and 12 are glued together by a thinadhesive intermediate layer 16. It is essential that this intermediatelayer have an acoustical impedance that is not substantially greaterthan zero. Acoustical impedance is a measure of the transmissability ofsound or shock waves through a material. It is defined as the sonicvelocity, i.e., the speed of sound in the particular material,multiplied by the material's specific gravity, i.e., the material'sdensity relative to water.

A low impedance adhesive is needed in order to insure that the inner andouter layers will function as a single acoustical unit. Also, a lowimpedance intermediate layer will limit the acoustical phenomenon knownas impedance mismatch. This phenomenon occurs when a wave frontpropagates from one material having one impedance to a second having adifferent impedance. The result of this pehnomenon is (l the distortionof the wave front, and (2) the generation of harmonic waves that oftenbounce back through the sur face material, while attempting to rend thelayers of material apart at their interfaces. While this phenomenoncannot, and to an extent should not, be eliminated entirely, it can becontrolled by l reducing the number of interfaces in any layeredcombination of materials, (2) minimizing the acoustical impedance of anyintermediate layers so that such layers will produce only minimaldistortion in the wave front, and (3) choosing inner and outer materialswith relatively similar acoustical impedances.

The present invention utilizes an outer layer whose tensile strength isbetween 10 and 10 pounds per square inch (psi), with a correspondingelastic modulus of between 10 and 10 psi. This range of strength ofmaterials covers the entire range of strengths taht could be consideredin designing tank armor. It is to be emphasized that in the combinationof layers that comprises the present invention, almost all knownmaterials possessing the one common property of sufiicient strength asdefined by the above-cited range of tensile strengths and elasticmoduli, would be suitable to use in the present improved armor. Thisstatement is subject only to the limitation that outer layer 10 of thecombination possess a greater strength and higher elastic modulus thanthe inner layer 12. This limitation derives from the basic concept ofthe invention which is essentially one of a relatively hard, outer layerfor absorbing the very high impulse initial impact of the munition 18,wherein the high impulse impact is transmitted to a deformable secondlayer whose acoustical characteristics can spread the shock wave over aspherical areas 20 (See FIG. 4) which is in the order of 100 timesgreater than the spherical area 22 which the outer layer, by reason ofits greater density, presents to the wave front. The effective result isa division of labor or a segregation of function, between two principallayers of armor, i.e., a harder outer layer whose main function is toslow down the incident munition, and a softer inner layer whose foremostfunction is to absorb the shock of the munition while the coursecompleting the armors function of stopping the munition. As aconsequence, a more efficient armor (wherein armor efficiency is definedas momentum halted per unit weight of armor) is obtained which meets alongfelt military need and represents as advance in its field.

The broad general classes of materials that could be used in ourimproved armor include fiberglass, graphite, plastic, metal and ceramic.Suitable metals would include steel, aluminum and titanium, as well asalloys composed of at least 30 percent of any of said metals. The use ofthese materials could be for either layer or both (in different forms),subject of course to the requirement of a deformable inner layer. Asaforementioned, a relatively close impedance match between layers isalso preferable. This match would require layers having an acousticalimpedance of within 5 grams/- centimeter of each other. The inventionwould however be operable with less than said impedance match, but theresultant armor efficiency would of course suffer. Also, it should benoted that a higher impedance in the outer of the two layers ispreferable.

It is to be noted that a military vehicle may be constructed wherein thedeformable inner layer would constitute an integral part ofa structuralouter member of the military vehicle. Such a construction would yieldfurther benefits of weight reduction.

Highest armor efficiency is obtained through the use of materials in theform of unidirectional woven fibers. Such fibers are usually heldtogether by the use of various resinous bonding materials that can besubstantially affect the characteristics of the fiberous material. Theresinous bonding materials include epoxy, polyester resin, polyurethane,polypropelene and nylon.

FIGS. 2 and 3 offer a microscope view of two materials that have beenfound suitable for armor use. FIG. 2 illustrates an epoxy bondedfiberglass having a tensile strength of IO psi and an elastic modulus of3.7 X l0 psi. The fiberglass fibers are denoted as element 24 and theepoxy bonding material as element 26. Each fiber 24 is surrounded by acoating 28 that serves to keep the fibers firmly impregnated in theepoxy. This epoxybonded fiberglass is suitable to use as the hard outerlayer of the invention. This type of fiberglass may be purchasedcommercially as SCOTCH-FLY 3M 1002 material. It is produced in thinsublayers that may be cross-plied for greater strength and then heat(330F) and pressure (60 psil-cured toform a single hard layer.Twentyseven of these sub-layers having a total thickness of aboutfive-eighths inch. have been used in forming a single outer layer. Onesuch outer layer exhibited an areal density of 6.46 lbs/ft.

FIG. 3 illustrates a material that may be used as the deformable innerlayer. Element 30 is a fiberglass fiber having a lower strength thanfiberglass 24 of the outer layer. This resinous bonding material is apolyesterresin 32 and the coating is a type of starch 34. This starchimparts to the structure the properties of deformability and shockabsorbability that are desired. The bonding material 32 of FIG. 3 isknown as PARA- PLEX and is commercially sold by Rohm and Haas asmaterial P43. This material is sold in liquid form and is used toimpregnate the fiberglass fabric fibers which have been coated withstarch, e.g., by spraying them with an aqueous starch solution. Layersof the impregnated fabric are stacked and the Paraplex is cured underheat and pressure in known manner to form a structure containing layersof fiberglass fabric bonded together in a matrix of polyester resin.About l5 sublayers may be bonded together to form a layer having athickness of about one-half inch, a tensile strength of 6 X 10 psi, anelastic modulus of 3.3 X 10 psi, and an areal density of 3.2 lbs/ft.

Hence, the combined areal density of the inner and outer layers is 9.66lbs/ft." which is less than half of the areal density of conventionalsteel armor (20 lbs/ftF).

A greater strength in the inner layer may be obtained if the interwovensublayers are assembled with each adjoining sublayer having its fiberaxis angularly displaced by from the fiber axis of each adjoiningsublayer.

The adhesive intermediate layer 16 which holds the inner and outerlayers together may be comprised of an adhesive glue, wherein saidlayers are clamped together at about 30 psi until the glue rigidly sets.

The present improved armor may be assembled either on a mold with thehardened assembly later adhered to the tank surface, or the armor may bebuilt up directly on the outer structural members of the tank using asublayer-by-sublayer approach, similar to a paper-mache process.

It is thus seen from the above that the objects set forth in the Summaryof the Invention are among those made apparent from, and efficientlyattained by, the device of the preceding description.

We wish it to be understood that we do not desire to be limited to theexact detail of construction shown and described for obviousmodification will occur to per sons skilled in the art.

Having described our invention, what we claim as new, useful andnon-obvious and accordingly secure by Letters Patent of the UnitedStates is:

l. A shock absorbent layered combination of materials, comprising:

an outer layer possessing a tensile strength of between l0 and I0 poundsper square inch and an elastic modulus of between 10 and 10 pounds persquare inch;

a deformable inner layer rigidly adhered to said outer layer, whereinthe material constituting said outer layer possess both a higher tensilestrength and a higher elastic modulus than the material constitutingsaid inner layer; and

an adhesive intermediate layer which rigidly holds said inner and outerlayers together;

wherein the materials of said outer and inner layers are fiberglass,bonded with a resinous bonding material selected from the groupconsisting of epoxy,

layer comprises epoxy bonded fiberglass having a tensile strength ofabout 10 psi and said inner layer comprises polyester resin bondedfiberglass having a tensile strength of about 6 X l0 psi.

4. The combination as recited in claim 3 in which said outer layer has athickness of about five-eighths inch and said deformable inner layer hasa thickness of about one-half inch.

k I l t t

2. The combination of claim 1 in which the ratio of the areal density ofsaid outer layer to the areal density of said inner layer is about twoto one.
 3. The combination of claim 1 in which said outer layercomprises epoxy bonded fiberglass having a tensile strength of about 105psi and said inner layer comprises polyester resin bonded fiberglasshaving a tensile strength of about 6 X 104 psi.
 4. The combination asrecited in claim 3 in which said outer layer has a thickness of aboutfive-eighths inch and said deformable inner layer has a thickness ofabout one-half inch.